Use new event store when displaying calendar

This is faster because the event store is not yet connected to the calendar so
it does not trigger an update each time an event is added.

issue8875
review264841002

tryton/gui/window/view_form/view/calendar_gtk/calendar_.py

@@ -88,11 +88,7 @@ class Calendar_(goocalendar.Calendar):
             if date:  # select the day of the current record
                 self.select(date)
 
-        if self._event_store:
-            self._event_store.clear()
-        else:
-            event_store = goocalendar.EventStore()
-            self.event_store = event_store
+        event_store = goocalendar.EventStore()
 
         for record in group:
             if not record[dtstart].get(record):
@@ -120,6 +116,7 @@ class Calendar_(goocalendar.Calendar):
             event = goocalendar.Event(label, start, end, text_color=text_color,
                 bg_color=bg_color, all_day=all_day)
             event.record = record
-            self._event_store.add(event)
+            event_store.add(event)
+        self.event_store = event_store
 
         self.grab_focus(self.get_root_item())